After the rolling operation rails produced by hot rolling of rail steel in appropriately calibrated rolls are cooled by air on cooling beds to room temperature. Nevertheless, due to different ratios of mass to surface in the rail head and rail base the rails are deformed during cooling. Consequently, in order to respect straightness requirements, these rails have to be straightened on a roll straightener and, if necessary, restraightened by means of a gag press (German handbook "Die Eisenbahnschiene" by Fritz Fastenrath, editors Wilhelm Ernst & Son, 1977, pages 113/114).
During roller straightening the total rail section is plastically deformed. But as over the total rail section these deformations have different values, there are caused internal stresses in the straightened rails. At the tread surface and at the lower surface of the rail base, which are submitted to the highest loads during operation, these internal stresses are positive in the longitudinal direction, i.e. there exist internal tension stresses (cf. page 37 of the above cited handbook).
Said internal stresses may reach up to 50% and more of the rail yield strength.
During operation these stresses are superposed by flexural tensile stresses caused by the wheels, and longitudinal tension stresses caused by cooling and contraction of the rails at low temperatures. Consequently, the internal tension stresses existing in the rails reduce the rupture strength of the rails in case of surface flaws, such as for instance fatigue cracks due to static or dynamic strains (Technische Mitteilungen Krupp, Werksberichte 39 (1981), pages 33 to 44).
In order to reduce internal tension stresses in the rail head and base portions, the rails may be stretch-straightened (DE-OS No. 32 23 346) or controllably cooled and laterally straightened (DE-PS No. 19 42 929). But these processes comprise technical difficulties (stretch-straightening), and in certain cases the straightness required for high speed public transport is not precisely adjustable (stretch-straightening, controlled cooling in connection with lateral straightening) so that for reasons of economy and practicability these processes were not successful.
A further process for reducing internal tension stresses in the rail head and base portions in a normalizing operation.
According to the technical measures for reducing internal stresses through normalizing at increased temperatures, described by Houdremont in "Handbuch der Sonderstahlkunde" editors Springer-Verlag 1956, pages 238-240, said process is a heat treatment by annealing the rails at a temperature ranging from about 200.degree.-700.degree. C. comprising subsequent slow cooling. Since the decomposition of internal stresses is caused by a flow process starting at higher temperatures and discharging the rail material, reduction of internal stresses to low residual values of about 20-60 N/mm.sup.2 is only ensured through a period of time sufficient for the operational sequence of the flow processes. For this reason, the annealing treatment of rails requires in practice a duration of several hours. Citation "Technische Mitteilungen Krupp", Company Reports 39 (1981), page 33, indicates a treatment duration of six hours at a temperature of 550.degree. C.
It is therefore an object of the present invention to suggest an efficient and uncostly heat treatment process for reducing internal stresses in the head and base portions of roller straightened rails, which may be integrated into the usual production cycle of rails.